1. Field of the Invention
The present invention relates to a method for recovering baseband signals of ATSC (Advanced Television Systems Committee) system, NTSC (National Television System Committee) system, PAL (Phase Alternating Line) system, VSB (Vestigial Side Band) system, and SSB (Single Side Band) system from RF signals, and more particularly, a method for recovering ATSC/NTSC/PAL/VSB/SSB baseband signals directly down from RF signals by Zero IF or Direct Down Conversion without through the IF—Intermediate Frequency and passband (IF) filtering.
2. Description of the Prior Art
In the past, a super heterodyne method for demodulation applied in ATSC, NTSC, PAL, VSB and SSB in which signals only occupy a portion of double sideband needs a sharp passband (IF) filter to remove the signal of an adjacent channel before recovering a modulated signal. In the modulation process of ATSC/NTSC/PAL/VSB/SSB according to the prior art, first the redundant half of a symmetrical signal in a frequency domain is removed by an L/C circuit or a saw filter. The advantage of removing the redundant half of the symmetrical signal is that the same channel bandwidth, can be sent twice.
Once the modulated RF signal is received by an antenna, the demodulation process of ATSC/NTSC/PAL/VSB/SSB according to the prior art starts by amplifying the modulated RF signal with a low noise amplifier (LNA). Subsequently a mixer is applied to transfer the frequency of the modulated RF signal to an intermediate frequency (IF), then a sharp passband (IF) filter is then used to filter out unwanted RF signals at frequencies adjacent to the frequencies of a selected channel. And then convert to the baseband signal and recover the original signal. One of the primary uses with this demodulation process is that the sharp passband (IF) filter is required to filter out unwanted RF signals. The sharp passband (IF) filter requires complicated circuitry and a multitude of components, and is power consuming. An Imaging Rejection Circuit may reduce the rejection requirement of a sharp filter, but the sharp filter is still needed in order to meet the high requirement of the adjacent channel rejection. Moreover, the imaging rejection is also a very complicated circuit specifically for wide channel distribution such as video transmission channel from 40 MHz to 800 MHz.
In recent years, the development of computer and communications related technologies have grown rapidly, therefore conventional demodulation technology used in systems such as QAM, QPSK, OFDM, GSM, CDMA no longer requires a sharp passband (IF) filter because they are implemented with a zero-IF (intermediate frequency) demodulation method. In a system implemented with the zero-IF demodulation method, a modulated RF signal is mixed with a sinusoidal wave of the carrier frequency of the modulated RF signal with mixers to generate I (in-phase) and Q (quadrature-phase) components. Because the modulated RF signal is mixed with the sinusoidal wave of the carrier frequency, the I and Q components will be baseband components. The sharp filter is implemented in Base Band frequency. Due the low frequency of Base Band signal, the sharp Base Band filters are more easily implemented than prior art and can be integrated in to an Integrate Chip or a Mixed IC. This saves a lot of circuitry and components for QAM, QPSK, OFDM, GSM, and CDMA system. But thus far, demodulations in the ATSC system, NTSC system, PAL system, VSB modulation system, and SSB modulation system which only contain a portion of double baseband signal are still implemented with sharp passband (IF) filters. Thus they need much more complicated circuitry, external components such as SAW and are very power consuming, and thus cannot be easily integrated onto an SOC (system-on-chip).